A simple way to generate a checksum of a db.Model

util.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

__author__ = "Sahid Orentino Ferdjaoui"
__license__ = "Apache License, Version 2.0"

"""A simple way to generate a checksum from an instance of db.Model.

Note:
  These function run without dependences.

An example:
>>> class Actor(db.Model):
...   name = db.StringProperty()
...   age = db.IntegerProperty()
... 
>>> u = Actor(name="John Doe", age=26)
>>> util.checksum_from_model(u, Actor)
'-42156217'
>>> u.age = 47
>>> checksum_from_model(u, Actor)
'-63393076'

"""

def checksum_from_model(ref, model, exclude_keys=[], exclude_properties=[]):
    """Returns the checksum of a db.Model.

    Attributes:
      ref: The reference og the db.Model
      model: The model type instance of db.Model.

      exclude_keys: To exclude a list of properties name like 'updated'
      exclude_properties: To exclude list of properties type like 'db.DateTimeProperty'

    Returns:
      A checksum in signed integer.
    """
    l = []
    for key, prop in model.properties().iteritems():
        if not (key in exclude_keys) and \
               not any([True for x in exclude_properties if isinstance(prop, x)]):
            l.append(getattr(ref, key))
    return checksum_from_list(l)

def checksum_from_list(l):
    """Returns a checksum from a list of data into an int."""
    return reduce(lambda x,y : x^y, [hash(repr(x)) for x in l])